There is an urgent need to develop effective therapeutic approaches against category B, tier 1 pathogens, such as Burkholderia mallei (Bm) and B. pseudomallei, the causative agents of human glanders and melioidosis. Concern over these bacteria has heightened because of the pathogens' seemingly perfect characteristics for malicious use as a biowarfare weapon against humans. A vaccine developed to combat these bacterial agents will also have value for the immunization of at-risk populations in melioidosis endemic areas of the world.
Burkholderia mallei are non-motile bacterium responsible for glanders. This disease mainly affects horses, which are considered to be the natural reservoir for infection, although mules and donkeys are also susceptible (Neubauer et al. 2005 Journal of Veterinary Medicine Series B 52:201-5). Humans are accidental hosts of B. mallei following prolonged and close contact with infected animals. B. mallei infect humans by entering through open wounds and surfaces of the eyes or nose. Symptoms of glanders are dependent on the route of infection (Srinivasan et al. 2001 N Engl J Med 345:256-8). B. pseudomallei are motile bacteria causing melioidosis (Dance 1991 Clin Microbiol Rev 4:52-60). Melioidosis is a life-threatening disease that is mainly acquired through skin inoculation or pulmonary contamination, although other routes have been documented. This saprophyte inhabitant of soil environments is mainly encountered in Southeast Asia and northern Australia, but is sporadically isolated in subtropical and temperate countries (Stone 2007 Science 317:1022-24).
Both Burkholderia species are highly pathogenic and are classified as such in list B by the Centers for Disease Control and Prevention (Horn 2003 Surgical Infections. 4:281-87). Burkholderia infections are difficult to treat with antibiotics and there are several reports that indicate it is feasible to protect against melioidosis, at least in animal models of disease, with non-living vaccines (Nelson et al. 2004 J Med Microbiol 53:1177-82). There has also been some progress in identifying partially protective subunits. Passively administered antisera raised against flagellin, polysaccharide, or conjugates of polysaccharide and flagellin, protect diabetic rats against challenge with B. pseudomallei (Brett et al. 1994 Infect Immun. 62:1914-19; Brett and Woods 1996 Infect Immun. 64:2824-28; Bryan et al. 1994 Can J Infect Dis. 5:170-78). However, B. mallei are not motile and do not produce flagella. Moreover, the ability of flagellin to induce protection against an aerosol, or intranasal challenge has not been reported. Therefore, flagellin was assessed as a potential candidate for inclusion in a Burkholderia vaccine and found unsuitable. In contrast, all of the current evidence indicates that other surface-expressed or secreted proteins are immunogenic and structural similarity exists between the proteins in B. pseudomallei and B. mallei (Whitlock et al. 2007 FEMS Microbial. Lett. 277:115-22; Whitlock et al. 2008 Transactions of the Royal Society of Tropical Medicine& Hygiene 102 Suppl: S127-33).
Burkholderia mallei, the causative agent of glanders, are Gram-negative, obligate mammalian pathogens. Glanders is primarily a disease of solipeds, with rare cases occurring among humans1-2. Naturally acquired human cases occur in endemic areas, particularly among those exposed to infected solipeds2-4. Additionally, cases have been reported among laboratory workers5-6. The World Organization for Animal Health coordinates ongoing efforts towards worldwide eradication; however, regional endemicity still exists in Africa, Asia, the Middle East, and South America2. Recent equid outbreaks in the Middle East and Asia7-8-9 set the stage for possible glanders reintroduction into disease-free regions and as a result glanders is classified as a re-emerging disease.
Glanders is a debilitating and often fatal disease transmitted via cutaneous and respiratory routes. Disease course and severity is route-dependent, with respiratory cases being the most severe3. Respiratory infection is characterized by rapid onset of symptoms, including fever, lymphadenopathy, pulmonary abscesses, pneumonia, disseminated organ infection, and ultimately septicemia3, 5, 10. Because of the high incidence of septicemia following respiratory infection, fatality rates in human respiratory cases have been estimated at 90% without treatment and 40% with aggressive antibiotic therapy11.
The use of B. mallei as a bio threat agent has been documented in different world military conflicts10, 12-13. Its amenability to aerosolization, low infectious dose, high case fatality rate, and high-level antibiotic resistance make B. mallei a top candidate for malevolent use1, 10, 14. Because of its perceived public health threat, the Department of Health and Human Services has categorized B. mallei as a Tier 1 Select Agent. The lack of effective treatments against these bacteria highlights the need for an effective vaccine. Numerous vaccine strategies have been tested; however, to date there are no approved vaccines and the search for a candidate that can provide sterilizing immunity has proven elusive.
Recently the subject inventors identified several proteins i.e., SimA, Hcp1 and BopA potentially capable of inducing a protective or therapeutic immune response to Burkholderia mallei or B. pseudomallei. (See U.S. Pat. No. 9,267,947, granted on Feb. 23, 2016). Notwithstanding the foregoing, other methods and compositions for use in protecting humans and animals (e.g., equine animals such as horses, donkeys, and mules) against the Gram-negative bacterial pathogens Burkholderia mallei and B. pseudomallei are desired.
The present invention addresses this need by providing novel attenuated Burkholderia mallei strains and vaccine compositions containing for use in treating and/or providing immunoprotection against infections elicited by Burkholderia mallei (Bm) and B. pseudomallei, especially human glanders and melioidosis.